State of the art electronic systems typically require high density, high speed, low loss interconnection networks. Often, programmable or reconfigurable interconnections are required, so that data may be routed or switched among large numbers of active devices. For example, high performance programmable interconnections are necessary for communication among processors in parallel processing arrays, and between chips, modules and boards in very large scale integration (VLSI) microelectronic systems. As is well known, conventional electrical interconnection systems such as electronic switching networks, are often limited in terms of speed, and density, and typically consume excessive power.
As a result of these limitations, the art has heretofore proposed using programmable optical interconnection networks instead of electronic networks. In an optical interconnection network, free space optical interconnects are substituted for electrically conductive interconnects. A comparison between electrical and optical interconnect performance may be found in a publication entitled Comparison Between Electrical and Free Space Optical Interconnects For Fine Grain Processor Arrays Based On Interconnect Density Capabilities, by inventor Michael R. Feldman et al., published on Sep. 15, 1989 in Applied Optics, Vol. 28, No. 18, page 3820-3829. Prior art approaches for programmable optical interconnections have used fiber optic elements, optoelectronic elements and/or lenses.
It has been proposed to use holograms as part of an optical interconnection network. See the above mentioned Feldman et al. publication which describes the use of a thin computer generated holographical optical element above a processing element plane to interconnect transmitters and detectors in the processing element plane. Faceted holographic optical elements may also be used. Another article by inventor Feldman et al. entitled Interconnect Density Capabilities of Computer Generated Holograms For Optical Interconnection of Very Large Scale Integrated Circuits, Applied Optics, Vol. 28, No. 15, Aug. 1, 1989, pp. 3134-3137, also describes the use of a single transmissive computer generated hologram and a reflective mirror in a double pass arrangement to form an optical interconnection. See also an article by Feldman et al. entitled Holograms For Optical Interconnects for Very Large Scale Integrated Circuits Fabricated by Electron Beam Lithography, published in Optical Engineering, Vol. 28, No. 8, August, 1989, pp. 915-921.
Other approaches for implementing optical interconnections have used holograms in combination with spatial light modulators. As is well known to those having skill in the art, a Spatial Light Modulator (SLM) may be formed of liquid crystal devices, PLZT devices, deformable mirror devices or other known devices. Spatial Light Modulators include binary phase and binary amplitude SLMs. In the binary phase SLM, an incoming light beam passes through the SLM with either zero phase shift or 180.degree. phase shift. In other words, the phase is either unaltered or is reversed. In a binary amplitude spatial light modulator, an incoming light beam passes through the SLM in unmodified form or is blocked by the SLM. It is also known that SLMs are available as discrete SLMs or as arrays of SLMs, with each SLM in the array often being referred to as a picture element or "pixel".
A number of prior art approaches have used SLMs in combination with holograms to provide optical interconnection networks. Unfortunately, to the best of the Inventor's knowledge, prior art SLM/holographic interconnection networks have been low speed, low density, low efficiency (i.e. they dissipate excessive power), unreliable, difficult to manufacture or combinations of these undesirable characteristics.
One example of an optical interconnection network using spatial light modulators and a holographic element is described in Optical Interconnections for VLSI Systems by Goodman et al., IEEE Proceedings, Vol. 72 No. 7., July 1984, pp. 850-866. This paper describes an optical interconnection technique, in which optical inputs are connected to optical detectors with programmable patterns. Patterns are changed using a dynamic mask (SLM) to block off undesired connections. Unfortunately, if this technique is used to connect an optical transmitter to any one of N detectors, only 1/N of the incoming light power would go to the desired detector.
Yet another optical interconnection network using spatial light modulators and holograms is described in an article entitled Dynamic Holographic Interconnects Using Static Holograms by Maniloff et al., Optical Engineering, March, 1990, Vol. 29, No. 3, pp. 225-229. A two-dimensional SLM array is used to address a fixed, multi-exposure, volume hologram. As is well known to those having skill in the art a thick or volume hologram diffracts an input wave into a single diffracted wave (in addition to the transmitted wave) when the input is incident at the "Bragg angle". If the input wave has an angle of incidence away from the Bragg angle, there is no diffracted wave and the energy is contained entirely in the transmitted wave. The characteristics of thick hologram were extensively analyzed by Kogelnik in the Bell System Technical Journal, Vol. 48, No. 9, November, 1969, pp. 2909-2947, in an article entitled Coupled Wave Theory for Thick Hologram Gratings.
As described in the Maniloff et al. publication, a ferroelectric liquid crystal amplitude modulated SLM is placed between cross polarizers to operate as an intensity modulator. A message from an originator processor modulates a laser, which illuminates a horizontal one-dimensional SLM or a row of a two-dimensional SLM. The SLM spatially encodes the address of the destination processor on the incident optical wavefront which then illuminates a static, multiple exposure hologram previously recorded in the volume medium. A reference beam is reconstructed at an angle associated with the address of the destination processor. Unfortunately, the Maniloff et al. technique requires the use of a thick, volume hologram which exhibits Bragg angle selectivity, so that a single outgoing beam at the Bragg angle may be produced, to impinge on a detector. As is well known to those having skill in the art, volume holograms are typically expensive and unreliable. Moreover, in order to interconnect a large number of input beams, multiple exposures of the volume hologram must be recorded. Unfortunately, it is difficult to record multiple exposures accurately, and to prevent the multiple exposures from interfering with one another.
Thin holograms are also well known to those having skill in the art. Thin holograms may be generated by computer, as opposed to multiple exposure optical recording techniques, using well known techniques. In a thin hologram, an input wave is diffracted into numerous output waves traveling in different directions. In other words, an input plane wave is converted into multiple output plane waves. Although the thin hologram is easy to manufacture, more reliable and less lossy, it could not readily be used in the above described Maniloff et al. system because the thin hologram does not exhibit Bragg selectivity.
Yet another approach for optical interconnection networks using holograms and spatial light modulators is described in U.S. Pat. No. 4,946,253 to Kostuck entitled Reconfigurable Substrate-Mode Holographic Interconnect Apparatus and Method. Kostuck uses a light source, a spatial light modulator and multiple holograms, which may be computer generated holograms. The spatial light modulator encodes information on the light beam and the polarization modulator polarizes the light beam. The SLM is used solely to encode information onto the light beam, and polarizers and polarization sensitive holograms are used for switching the light beam. Unfortunately, for large numbers of inputs or detectors, large numbers of serially coupled polarizers and polarization sensitive holograms would be required, resulting in difficult optical alignment and low efficiency.
Another reference describing an optical interconnection system using spatial light modulators and holograms is United States Statutory Invention Registration H738 to McManus et al., entitled Switched Holograms for Reconfigurable Optical Interconnect. McManus discloses an array of optical switches, each of which is a combination of a transmission SLM and a polarizing beam splitter. A single holographic plate is positioned above the output faces of the switch array, and includes a large number of subholograms of interconnection patterns. Unfortunately, McManus requires a large number of holograms and a large number of serially coupled polarization modulators to switch an incoming optical signal to an outgoing array. For example to switch N signals with P different possible permutations, NxP hologram facets and P serially coupled polarization modulators are typically necessary.
In conclusion, the above survey indicates there is a continued need for a high speed programmable or reconfigurable optical interconnection network which uses spatial light modulators and holograms efficiently, with minimal power loss and which can be manufactured reliably and at low cost.